Serial scan chain in a star configuration

ABSTRACT

A method implemented to test a plurality of components coupled in a star configuration, each component having a test access port (TAP) controller. The method comprises performing a capture phase of a scan operation on all of the TAP controllers in the star configuration and sequentially selecting one of the TAP controllers at a time to perform a shift state. When all of the TAP controllers have been sequentially selected to perform the shift phase, the method further comprises selecting all of the TAP controllers to perform an update phase.

RELATED APPLICATIONS

This application is a divisional of application Ser. No. 12/134,759,filed Jun. 6, 2008, currently pending;

Which claims priority to and incorporates by reference U.S. ProvisionalPatent Application No. 60/933,649, filed Jun. 7, 2007, entitled“Tracking Participants In A Multi-Drop Communications Scheme WhereMultiple Parties Parts May Be Active At Once,” which is incorporatedherein by reference in its entirety.

BACKGROUND

As electronic circuits and devices have become more complex, testing ofthese devices has become increasingly difficult. Test standards havebeen developed to address at least some of these testing difficulties.One such standard, written by the Joint Test Action Group (“JTAG”), isIEEE standard number 1149.1, which describes the Standard Test AccessPort and Boundary-Scan Architecture. Boundary scan is a methodology thatallows controllability and observability of the boundary pins in aJTAG-compatible device via software control. This capability allowstesting of circuit boards that otherwise might not be practical orpossible given the trace pitch and multi-layering of printed circuitboards today.

Typically, JTAG-compatible devices (e.g., chips, circuit boards) areconnected in series in a scan chain. However, it may be desirable toconnect such devices in a parallel (star) configuration for testingpurposes. Performing JTAG testing on devices in a star configuration isproblematic in the JTAG standard.

SUMMARY

In accordance with one embodiment, a method is implemented to test aplurality of components coupled in a star configuration, each componenthaving a test access port (TAP) controller. The method comprisesperforming a capture phase of a scan operation on all of the TAPcontrollers in the star configuration and sequentially selecting one ofthe TAP controllers at a time to perform a shift state. When all of theTAP controllers have been sequentially selected to perform the shiftphase, the method further comprises selecting all of the TAP controllersto perform an update phase.

In accordance with another embodiment, a system comprises a debug testsystem (DTS) and a plurality of target systems (TS) coupled to the DTSvia a test communication link in a star configuration. Each TS has atest access port (TAP) controller coupled to the test communicationlink. The DTS causes all of the TAP controllers to concurrently performa capture state in which all of the TAP controllers acquire new data andalso to transition to a pause state. The DTS sequentially selects oneTAP controller at a time to shift data out of the selected TAPcontroller while the other TAP controllers remain in the pause state.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of exemplary embodiments of the invention,reference will now be made to the accompanying drawings in which:

FIG. 1 shows a system in accordance with various embodiments;

FIG. 2 shows a state diagram usable in conjunction with the system ofFIG. 1;

FIG. 3 illustrates a series scan configuration;

FIG. 4 illustrates a star scan configuration in accordance with variousembodiments; and

FIG. 5 shows a preferred embodiment of a method in which a series scanis synthesized on a star scan configuration.

NOTATION AND NOMENCLATURE

Certain terms are used throughout the following description and claimsto refer to particular system components. As one skilled in the art willappreciate, companies may refer to a component by different names. Thisdocument does not intend to distinguish between components that differin name but not function. In the following discussion and in the claims,the terms “including” and “comprising” are used in an open-endedfashion, and thus should be interpreted to mean “including, but notlimited to . . . ” Also, the term “couple” or “couples” is intended tomean either an indirect or direct electrical connection. Thus, if afirst device couples to a second device, that connection may be througha direct electrical connection, or through an indirect electricalconnection via other devices and connections.

DETAILED DESCRIPTION

The following discussion is directed to various embodiments of theinvention. Although one or more of these embodiments may be preferred,the embodiments disclosed should not be interpreted, or otherwise used,as limiting the scope of the disclosure, including the claims. Inaddition, one skilled in the art will understand that the followingdescription has broad application, and the discussion of any embodimentis meant only to be exemplary of that embodiment, and not intended tointimate that the scope of the disclosure, including the claims, islimited to that embodiment.

FIG. 1 illustrates a system 10 in accordance with various embodiments.The system 10 comprises a debug and test system (DTS) 11 that is coupledto one or more target systems (TS) 16. The DTS 11 provides for testingof the DTS's 11. In some embodiments, the DTS 11 and TS's 16 areJTAG-compliant (Joint Test Action Group) and are coupled via a JTAGcommunication link 12.

The DTS 11 comprises a JTAG adapter 14 coupled to host logic 8containing a test access port (TAP) controller 13 (TAPC) via an IEEE1149.1 bus 15. Similarly, each TS 16 comprises a JTAG adapter 17 coupledto scan test logic (STL) containing a TAP controller (TAPC) 18 via anIEEE 1149.1 bus 19. DTS 11 is capable of sending test and debugsequences via link 12 to any or all of the TS's 16. The sequences allowDTS 11 to configure the TS's 16 for a test, execute the test, and readback the results of the test. The DTS 11 may be configured to couple tothe TS's 16 using a four or five wire implementation of link 12 asdefined under the JTAG architecture. The link 12 includes signals TCK(clock), TMSC (mode select), TDI (data in), TDO (data out), andoptionally RTCK (return clock). At least the TCK, TMSC, TDI and TDOsignals are used when the DTS 11 communicates with the TS's 16 accordingto the JTAG protocol. This architecture is further described in U.S.Pat. Pub. 20060279439, incorporated herein by reference.

FIG. 2 shows a state diagram in accordance with the JTAG standard. Fromthe test logic reset state, a TAP controller may enter the run test idlestate depending on the TMS bit. From run test idle, the TAP controllermay perform a data register (DR) scan or an instruction register (IR)scan. Each of the DR and IR scans generally involves the TAP controllerperforming a capture, shift, and update as illustrated. Various pause orexit states are built into the state diagram. Following the update, theTAP controller may return to the run test idle state or perform anotherDR or IR scan.

JTAG-enabled systems have generally been configured in a series scanconfiguration during testing as illustrated in FIG. 3. FIG. 3illustrates three target systems (TS 16 a-16 c). All three TS's arecoupled in series. The TDI signal from the DTS 11 is provided to TS 16 aand the TDO output of TS 16 a is connected to the TDI input of TS 16 b.The TDO output of TS 16 b is connected to the TDI input of TS 16 c. TheTDO output of TS 16 c provides the test output signal back to the DTS11. In a series scan configuration as in FIG. 3, all of the TAPcontrollers of the TS's 11 transition together through the state diagramof FIG. 2. That is, all of the TAP controllers of the TS's perform acapture, shift and update concurrently. All of the TAP controllers shifttheir data in unison with the output of one TAP controller being shiftedto the input of the next TAP controller in series.

FIG. 4 illustrates a star configuration in which the three TS's 16 a-16c share the JTAG signals TCK, TMS, TDI and TDO. In a star configuration,all of the TAP controllers of the TS's can perform capture and updatescans together, but not shifts. The reason that shifts cannot beperformed in unison is that all of the TS's would attempt to drivedifferent bits onto the same, common TDO signal line thereby creating adrive conflict on TDO. The embodiments disclosed herein solve thisproblem by implementing a series equivalent of the scan operation on astar configuration.

The operations performed by the Capture-xR and Update-xR TAP states (the“x” refers to either “D” for data or “I” for instruction) for a seriesscan topology (FIG. 3) are duplicated in a star scan topology (FIG. 4),with the number of clock periods between the Update-xR and Capture-xRTAP states remaining the same. However, the scan operation exchangingdata with the TAPs of interest between the Capture-xR and Update-xR TAPstates altered. The scan operation used with a series scan topology issynthesized using a series of scan operations with a star scan topology,each operation in the series targeting a single and different TAPcontroller of interest. A description of this process follows.

In a star scan topology the TAP controllers of interest (TAP controllersto be scanned) are called the “Scan Group” while the TAP controllersthat are not of interest (TAP controllers not to be scanned) are calledthe “Idle Group.” The TAP state of the Idle Group is parked in theRun-Test/Idle state while the Scan Group participates in DR and IRScans. A series equivalent scan is performed with the Scan Group asfollows.

Referring to FIGS. 2 and 5, the TAP state of the Scan Group movesthrough the Capture-xR and Update-xR TAPC states 102 and 118 inlock-step. That is, in those states, all members of the Scan Groupperform captures and updates in unison. Between the two states 102 and118, the shift portion of a scan is divided into sections with eachsection dedicated to selecting and scanning only one member of the ScanGroup while the TAP state of the other members of the Scan Group areparked in the Pause-xR state.

At 102, all members of the Scan Group perform a capture which entailsmoving through the Select-xR, Capture-xR and Exit1-xR states, withouttraversing the Shift-xR TAP controller state.

At 104, all members of the Scan Group transition to the Pause-xR state.While in the Pause-xR state, a Scan Selection Directive (SSD) isperformed. Various SSDs are possible and are encoded bits on the TDIinputs to each TS 16. One of the SSDs is a “select one” TAP controllerin which one of the TAP controllers is selected using an address uniqueto the target TAP controller. Another SSD is a “select all” SSD in whichall members of the Scan Group are selected. At 104, a “select one” SSDis performed to select one member of the Scan Group (e.g., the TAPcontroller of TS 16 a). With the TAP controller of TS 16 a selected andthe other TAP controllers not selected (decoupled), a shift is performedat 106 on the one selected TAP controller. The shift is performed bymoving the selected TAP controller through the Exit2-xR state, theShift-xR state, the Exit1-xR state and finally back to the Pause-xRstate.

At this point, all members of the Scan Group are in the Pause-xR state.Another one of the TAP controllers (e.g., the TAP controller of TS 16 b)is selected at 108 using the “select one” SSD message encoded with thetarget TAP controller's address. The selected TAP controller thenperforms a shift at 110 by moving through the Exit2-xR, Shift-xR,Exit1-xR and Pause-xR states. At 112 and 114, yet another member of theScan Group (e.g., the TAP controller of TS 16 c) is selected using the“select one” SSD and then shifted as described above.

Once all members of the Scan Group have been individual selected andshifted, at 116, a “select all” SSD is issued to select all members ofthe Scan Group while in the Pause-xR state. At 118 (and all members ofthe Scan Group have been selected and coupled), the Update phase of thescan is performed by moving the Scan Group through the Exit2-xR andUpdate-xR states and, if desired, back to the Run-Test/Idle state.

The “select one” SSD uses a value that is unique to the targeted TAPcontroller. That value can be a TAP Controller Address (TCA) or aController Identifier (CID). A TCA preferably is a 35-bit valuecomprising a 27-it IDCODE concatenated with an 8-it node identificationnumber (NODE ID). Per the IEEE 1149 specification, the 27-bit IDCODEcomprises a 16-bit part number and an 11-bit manufacturer identifier.The 8-bit NODE ID provides for 256 uniquely addressable TAP controllerswith identical IDCODE elements. The NODE ID is created at the chip levelusing any one of several methods or a mix of the following methods: fromexternal pins whose value is latched when chip hard reset is released,fusible elements, programmable elements such as electrically-erasableprogrammable read-only memories (EEPROMs), a register loaded by theapplication, and fixed (hardwired). The TCA is generated and provided tothe associated TAP controller.

In alternative embodiments, since TCAs are 35 bits in length and atleast some systems will have 16 or fewer TAP controllers, a performanceimprovement can be attained by allocating a four-bit alias for up to 16TCAs. This alias is called a Controller ID (CID). Any of a variety oftechniques is possible to allocate CIDs to each TAP controller.

The SSDs described above can use either the TCA or CID values to selecta particular TAP controller. SSDs are encoded with a 3-bit directive tospecify the type of SSD. A directive bit pattern of “010” means that theSSD is a “select one” SSD using a CID. The CID value is part of theSSD's payload. A directive bit pattern of “110” means that the SSD is a“select one” SSD using a TCA, which is part of the SSD's payload. Adirective bit pattern of “111” means that the SSD is the “select all”SSD. A directive bit pattern of “000” means that the SSD is a “clearall” SSD (all members of the Scan Group are decoupled).

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. It is intended that the followingclaims be interpreted to embrace all such variations and modifications.

1. A process of testing target systems connected in a starconfiguration, each target system having a test access port controller,comprising: A. moving all of the TAP controllers from a RunTest/Idlestate, through a Capture-DR state and an Exit1-DR state into a Pause-DRstate without traversing a Shift-DR state; B. selecting one of the TAPcontrollers with encoded bits on a TDI input to each target system; C.moving only the selected TAP controller through an Exit2-DR state, aShift-DR state, the Exit1-DR state, and back to the Pause-DR state; andD. de-selecting the one TAP controller and selecting another one of theTAP controllers with encoded bits on a TDI input to each target system;E. moving only the other selected TAP controller through an Exit2-DRstate, a Shift-DR state, the Exit1-DR state, and back to the Pause-DRstate; and F. selecting all of the TAP controllers with encoded bits ona TDI input to each target system and moving all of the TAP controllersfrom the Pause-DR state to an Update-DR state.
 2. The process of claim 1in which selecting one of the TAP controllers includes generating acontroller address unique to the TAP controller.
 3. The process of claim1 in which selecting one of the TAP controllers includes providing acontroller address to all of the TAP controllers, the controller addressbeing valid for only one of the TAP controllers receiving the address.4. The process of claim 1 including assigning a unique address oridentifier to each TAP controller from which each TAP controller is thenselected.